Integrated construction of low-cost gas lasers

Integrated construction of low-cost gas lasers

Michael E. Fein and Charles W. Salisbury

Helium-neon lasers have been fabricated using methods and materials particularly suited to high-volumein-line production. Pilot-project results suggest that satisfactory life and performance can probably beachieved in production lasers incorporating pressed or ground soft-glass parts, extensive solder-glass seals,and thin-film-metallization cathodes.vative laser products.

Introduction

The development of such laser applications as thevideo-disk player and the point-of-sale scanner maysoon create sufficient demand for low-cost lasers towarrant the manufacture of radical new gas laser de-signs, particularly suited to in-line high-volume pro-duction. We describe here one family of new designs,which resulted when a research and development groupexperienced in gas-discharge displays set out to developgas lasers. Our basic idea was to fashion a laser not byflame-working glass tubing, but by joining slabs orpressed shapes of glass with low-melting solder glasses.We refer to the class of devices we have worked with asintegrated lasers, because the method allows many ofthe components of a laser to be fabricated in or upon asingle substrate.

An illustration of the concept is the laser substrateshown in Fig. 1. This flat glass laser body was made bymachining several cavities in a thick piece of glass, usingconventional glass-grinding equipment.

After numerous experiments with flat glass lasers hadtaught us some rules for effective device design, weconducted most of our later experiments with pressedglass parts (cf. Figs. 2-4) in which part of the necessarystructure is provided by molding the basic glass part.The pressing procedure was little different from thatused for glass ashtrays.

It seems reasonable that any particular laser cavitygeometry that might be built by conventional tubularmethods can be fairly well approximated in an inte-grated laser, so that we have not concerned ourselvesinitially with such matters as design for optimum poweroutput. All the devices shown are on the order of 30 cmlong by 15 cm wide and have active bores about 1 mmby 15 cm.

The authors are with Owens-Illinois, Inc., P. 0. Box 1035, Toledo,Ohio 43666.

Received 3 January 1977.

The new construction methods suggested should lead to other inno-

Our principal problem has been to demonstrate thatseveral materials questions would have satisfactoryanswers. Specifically, we asked:

(1) Could pressed (soda-lime glass) parts be used todefine, in a single substrate, the entire geometry of along-lived laser? (This worked.)

(2) Could low-melting solder glass be used to sealtogether the entire laser? (This required solder glassto be used close to the gas discharge, which has not beendone in prior lasers. With due precautions, thisworked.)

(3) Could thin-film metallizations be used for cathodeand anode? (With proper geometric design, thin-filmcathodes worked. We have not yet made successfulthin-film anodes.)

We now feel that the integrated construction ap-proach holds considerable promise for the productionof inexpensive gas lasers in very large volume.

Laser BodyFigure 2 shows an assembled laser. We have clearly

made no effort, at this point, to make the laser partic-ularly lightweight or compact. The goal of this programwas to demonstrate the workability of the technologiesused.

Figure 3 shows the pressed substrate from which thelaser was made. This pilot model was made on a handpress of a type commonly used to test prototypes forhigh-volume pressed ware. The glass used was a com-mon soda-lime glass of a variety that would be readilyavailable at a pressing plant. The primary lesson fromthis prototype-pressing experience was that the broadflat areas incorporated in the initial design were diffi-cult, although not impossible to press. (There was atendency of the molten glass gob to cool before it spreadto fill the entire mold.) Since a high-production devicewould have a more compact design in any event, this isnot a serious problem.

The pressed part contains a cathode cavity and acathode-connecting channel. Experiments were also

2308 APPLIED OPTICS / Vol. 16, No. 8 / August 1977

Fig. 1. Integrated laser substrate made by grinding depressions ina 2.2-cm thick float glass slab. A flared-end glass tube (not shown)is to be sealed into the round-bottom groove entering the long sideof the cathode cavity to carry the discharge into the center of the

cathode volume.

(a)

(b)

Fig. 2. Assembled pressed-glass laser on test stand.

Fig. 4. Pressed-glass laser substrate after preliminary processing.

conducted by Robert Knisely and Robert Bieringer ofOwens-Illinois in which they attempted to develop amethod for pressing the laser bore as well. Their initialefforts were unsuccessful, in part because the narrowridge needed to mold a 1-mm groove is subject to over-heating and rapid deterioration during the moldingprocess. Direct pressing of the groove would be morestraightforward for larger-bore lasers, such as the CO2laser.

In Fig. 4, the pressed part is seen after several stagesof processing:

(1) The sealing surface has been flattened by grind-ing. (The roughened surface left by grinding accountsfor the fuzzy appearance of the back of the part, as seenthrough the front surface.) In the longer term we wouldconsider flattening the surface by sagging, which shouldbe less costly.

(2) A stepped hole has been drilled for a vacuum-processing tubulation using a diamond-impregnateddrill.

(3) Another stepped hole has been drilled for inser-tion of an anode pin. The inside section of this hole isabout 6.4 mm wide and 3.2 mm deep to provide spacearound the anode pin for the discharge to surround itand make good electrical contact.

August 1977 / Vol. 16, No. 8 / APPLIED OPTICS 2309

Fig. 5. Float-glass mating substrate for pressed-glass laser aftermetallization. (The gray background and the fold at upper left are

artifacts introduced by the photographer.)

(3) The cathode metallization. (A mating metalli-zation was simultaneously put on the pressed sub-strate-it is visible in Fig. 2.)

(4) Raw cut edges in glass parts have proven, pre-dictably, to be weak points in finished devices. Theedges of cut slabs of glass have therefore been givenpencil-edge grinds (a soft, rounded contour), and thegrooves cut for laser bores have consistently been acidpolished to remove microfissures that could initiatebreaks.

Figure 6 shows the float-glass part after further pro-cessing; a devitrifying solder glass (Owens-Illinois typeS89415) has been screen-printed onto the part andburned out to remove the vehicle. Here the part is seenfrom the inside. Note that the conductive cathode tabvisible in Fig. 5 passes under the solder glass to makeexternal electrical contact.

To arrive at the assembled device shown in Fig. 2, thefollowing additional steps were necessary:

(1) An anode pin (52 iron-based sealing alloy) wassolder-glass-sealed into its hole in the pressed part. We

Fig. 6. Float-glass part after seal burnout.

(4) Shallow depressions (about 1.8 mm wide by 0.6mm deep) have been ground where the passive boresections will be. These depressions enlarge the passivebore sections so that diffraction loss is dominated by theactive bore. The passive end sections are needed onlyto protect the laser mirrors from damage by the gasdischarge.

Figure 5 shows the mating section of the laser body,which was manufactured from 6.4-mm float glass.Details visible in this view, from the outside of thesubstrate, include:

(1) The active laser bore (nominally 15.2-cm long,1-mm square cross section). This groove, like theothers, was cut with a standard diamond-impregnatedgrinding wheel.

(2) The enlarged passive bore sections at the ends.Note the convenience of creating such complexities

in bore design-we simply needed to run the substrateunder an additional tool. The analogous process in atubular laser would be to butt seal two pieces of tubingof different diameters, with considerable care taken tomaintain coaxial alignment.

(a)

(b)

Fig. 7. Alternative constructions. Figure 7(a) illustrates a Brewsterwindow, a vacuum-processing tubulation sealed in a groove in onesubstrate, and a platinum-foil anode. Figure 7(b) shows a three-partglass body and a tubular-aluminum cathode. Mirrors were yet to be

added to the device shown.


experimented briefly with a thin-film aluminum anode,which burned through quickly when hit by the gas dis-charge. The small area in which all the anode currentwas concentrated, as is normal in laser construction,makes this burn-through unsurprising. We hope thatfurther development will allow us to spread out thecurrent, to reduce the energies of particles incident onthe anode, or simply to use a more durable thin-filmmaterial, so that the thin-film anode remains a possiblealternative. We have made successful devices using athin platinum-foil anode sealed between the glass parts[see Fig. 7(a)], which lends encouragement to the pos-sibility of a long-lived thin-film anode.

(2) A vacuum-processing tubulation was solder-glass-sealed into the stepped hole shown in Fig. 4.

(3) The substrates were sealed together in an oven.(4) Mirror-mounting surfaces were ground on the

ends of the assembled laser body.(5) Mirrors were aligned and mounted with jigs and

epoxy systems similar to those being used at the timefor O-I's tubular laser designs.

The pressed-glass laser should also be compatiblewith the solder-glass mirror seals developed for 0-I'stubular lasers. Since these seals were developmentalat the time we began building pressed-glass lasers, wedecided not to use solder glass in our early tests. Useof solder glass to seal mirrors will eventually be desirableto achieve better hermeticity and also to permit bakeoutto higher temperatures during gas processing.

(6) A thermocouple pressure gauge (HastingsDV16D) was added by a glassblower to monitor gaspressure during life testing.

CathodeThere should be no difficulty in constructing inte-

grated lasers with conventional sheet-metal cathodeinserts [cf. Fig. 7(b)], but we chose to take advantage ofthese open, easily coated laser body components byattempting to make long-lived thin-film cathodes. Theuse of thin-film cathodes should become advantageousparticularly as production volumes become high enoughto justify fully automated in-line manufacturingequipment. Several design features appear to be im-portant to successful cathode design:

(1) cathode geometry designed to minimize peakcurrent density;

(2) proper cathode material;(3) protection of metallization edges;(4) tapered cathode-connecting volume.These features are discussed in this section.Hochuli has suggested that peak cathode current

density be limited to 600 gA/cm2 for stable long-termoperation of solid aluminum-alloy cathodes undertypical operating conditions.' A more conservative 100,gA/cm2 has been suggested'.by Goldsborough.2 Thepressed-glass design shown in Figs. 2-6 has about 110cm2 of coated area, which if current were uniformlydistributed would give 45 giA/cm 2 at 5-mA total current.This design is probably more conservative than neces-sary, but careful experimentation will be needed to es-tablish just how small a cathode can be used.

(a)

(b)

Fig. 8. Experimental device with too small a cathode-connectingchannel and too shallow a cathode volume: (a) full view; (b) closeup

of severe cathode erosion.

It is not sufficient to provide a lot of cathode area.Cathode geometry must be such as to promote reason-ably uniform spreading of cathode current. One im-portant geometrical aspect is adequate spacing betweenopposing cathode walls. Figure 8 shows an early ex-perimental device in which the spacing between top andbottom cathode surfaces was too small. Perhaps be-cause of strong development of the hollow-cathode ef-fect (cf. Ref. 3, pp. 97ff), the current concentrated in asmall region near the entrance to the cathode andquickly sputtered through the metallization.

One advantage, in fact, of using thin-film electrodesis that this type of design deficiency becomes quicklyevident. If sputtering of cathode material is at all sig-nificant, a hole quickly appears in the cathode to signalthat redesign is necessary.

While it is not yet clear what minimum plate spacingis necessary for typical He-Ne laser currents, we sawgood performance with the 2.5-cm depth in the modelshown and also with a 1.8-cm spacing in an earlier flat-glass device.


The cathode-connecting channel provides room forthe discharge to spread from the 1-mm 2 cross sectionof the positive column to about 6.4 cm2 where it entersthe cathode volume. Note in Fig. 2 that the edge of themetallization is concave with respect to the dischargeentry. This feature allows the discharge to spreadfurther before hitting metal. The particular metalli-zation-edge design shown was evolved experimentallyby observing where current appeared to concentrate inearly devices and trimming the deposition masks untila reasonably uniform current distribution was ob-tained.

An interesting operating feature of the mask shapethat evolved is that the positive column ends just at theend of the cathode-connecting channel, and the Faradaydark space appears rather precisely to fill the spacebetween the channel's end and the metallization edge.Further experimentation is needed to see if this remainstrue with deeper or shallower indentations in the me-tallization and whether discharge stability, operatingvoltage, and cathode erosion rate are significantly af-fected by such variation.

The generally circular shape of the cathode cavity wasan attempt to get as much metal close to the dischargeentry as possible. (The ideal application of this designcriterion would be to have the discharge enter at thecenter of a sphere.) It is well known4 that in generallyelongate cathodes the current tends to concentrate nearthe entry, thus increasing the likelihood of sputteringin this area.

We take the uniform appearance of the glow that fillsthe cathode during operation to be a sign that thecathode design shown is reasonably well optimized.Current is apparently being spread rather uniformly.As we work to develop smaller, higher-current-densitycathodes, current distribution will need to be measuredaccurately by dividing the metallization into indepen-dently wired segments.

The aspect ratio of the tapered cathode-connectingchannel has generally been kept less than about 2:1.We found in early experiments, with wide shallowchannels, that the discharge tended to contract to aroughly uniform diameter equal to the smaller channeldimension and to move around unstably in the largerdimension. This is an unacceptable source of dischargenoise. The small-aspect-ratio channels do not appearto have this problem.

Figure 1 shows the cathode of an early flat-glassprototype device, which was fabricated entirely bygrinding channels in a glass slab. This device illustratesa different approach to the problem of getting currentto spread out well over the cathode. The discharge iscarried approximately into the center of a cathodevolume by a bent tube with a flared end, which is sealedinto the round-bottomed portion of the cathode-con-necting channel. The center-entry position appears tobe more effective in spreading current than the end-entry position normally used in tubular construction.In our integrated-construction lasers, center entry is notparticularly harder to achieve than end entry, althoughit does require a somewhat wider substrate.

In keeping with the common belief that the edges ofmetal components are the most vulnerable to sputter-ing, we designed our cathode metallizations so thatedges are hidden by the solder glass seals (cf. Fig. 6)wherever possible. At the current densities used so far,this precaution appears unnecessary, since the exposedmetal edge at the discharge entry is not eroding. Athigher current densities, we contemplate protecting thatexposed edge with an insulative coating.

An important feature of the design of the cathodecavity is that the slopes of the edges are relativelygradual (450 in the devices shown in this paper). Thishelps to obtain a reasonably uniform metallizationduring evaporative coating. Rounding of all edges en-sures the continuity of metallization across thoseedges.

The cathode is made by electron-beam evaporation;a 200-A adhesion layer of chromium is followed by 3 gmof aluminum. (It is not clear that so thick a metalliza-tion is needed.) E. Lee Williams of Owens-Illinois hasalso manufactured successful cathodes for tubularHe-Ne lasers by filament evaporation of between 0.2gm and 0.3 gm of pure aluminum with no adhesionlayer. As of 8 September 1976, one such device had runfor 13,800 h at 5 mA and was still operating well.Cathode area of this laser was 77 cm2, and gas storagevolume was about 60 cm3. Williams's work, which hewill report separately when a life test program is com-plete, establishes with greater certainty that thin filmscan fulfill the promise suggested by our initial experi-ences using them in integrated lasers.

We have contemplated a number of techniques formaking external electrical connection to the cathodefilm. In the experimental devices reported here, we cutour evaporation masks so as to extend thin-film con-ductive tabs under the solder glass seal that surroundsthe cathode. Those tabs were extended to the outsidesurfaces of the substrates with a silver thick film paste,so that a metal pressure clip could be connected. Thethick film pad and clip are visible in Fig. 2(b).

Solder-Glass SealIt is not novel to have used solder glass in a He-Ne

laser. Several manufacturers have developed the ca-pability to use solder glass to seal mirrors in order toimprove hermeticity in comparison to the more commonepoxy seals. The way in which solder glass is used inour device is unusual, however, which led us to be con-cerned about several possible modes of failure:

(a) We use a lot of solder glass so that any problemswith outgassing or with virtual leaks will be magni-fied.

(b) The solder glass in our laser is exposed to ratherhigh fields.

(c) The discharge, which is a source of reactivespecies, comes closer to the solder glass in our systemthan when solder glass is only used for mirror seals.

There is precedent for our concern. A class of defectsin electron tubes has been traced to electrolysis of softglasses under high-field conditions.5-7


Figure 9 shows an intriguing dendritic growth patternthat tends to appear in the solder glass when dischargecontact is allowed. Similar patterns have been observedin electrolytic decomposition of glass in ellectrontubes.7

Gas Processingour gas processing schedule was much like that used

glass safor tubular gas lasers8: oxygen discharges to help re-move organic impurities; a series of flushes with He-Ne

denacmixture; a high-temperature bakeout; and a final series_t~c /r~ \* Z created byof He-Ne discharges to reach maximum laser power.

The processing cycle used was by no means optimizedfor speed of production. Our objective at this stage wassimply to make a few long-lived lasers. A few notes arein order on the details of our gas-processing cycle.

ressed glass laser exhibiting dendritic growths in the solder (1) With the thin-film cathode, there is no need forglass seal. the high-current oxygen discharge commonly used to

outgas thick-metal cathodes. It is not entirely clear, infact, that oxygen is needed at all. Detailed experimentswill be needed to see if the thick surface oxide suppos-edly created by such oxygen discharges is really neededfor long life, when the inherently clean thin-film cathodeis used. Hochuli's results suggest that surface oxide

a _________________________ _ ( ) Thelps retard gas cleanup, but just how thick an oxide isneeded remains unclear.

(2) Because of the large volume of soft glass in ourpilot model, slow heating and cooling were called for inthe bakeout cycle to avoid thermal shock. We conser-vatively chose -1.1 0C (2F)/min as our maximum rate.

500 10,000I I I I I I I I With a more compact and lightweight design and withsome experimentation to establish minimum cycletimes, these rates would become much faster.

(3) The gas-processing station available to us wastil ife history of pressed laser (a)po relatively leaky, although we gradually improved itCHANGE METER during the course of the project. It became dramati-

cally clear that laser life was strongly dependent onMETER REPAIRED system cleanliness, so that we regard the life results

achieved as being subject to improvement with at rebuiltvacuum system.

Final fill before seal-off was with 2.7 Torr of a 7:1I I I 1 500 I I I I I601I1 1,0 He-Ne mixture (measured with an MKS Baratron).

TI000 HOURS 500 Natural isotopes were used, since at, this stage we hadno interest in the performance optimization that can be

Partial life history of pressed-glass laser PGL 12: (a) power obtained with isotopically enriched mixtures. Noteb) current; (c) anode voltage; (d) pressure indicated by that the indicated pressures in Fig. 10 are high by aboutgauge (actual pressure is about 2X lower). Pressure fluc- a factor of 2, because our Hastings thermocouple gauges)etween 5000 h and 10,000 li are probably due to an errati-

_11_ --- are calibrated for air.

As it turned out, we now feel confident that solderglass can safely be used in an application such as ours,so long as direct contact with the discharge is avoided.Where the discharge does touch the solder glass, re-duction apparently occurs-the surface of the solderglass blackens and appears lead-rich under Augerspectrometer analysis. We have found that it appearsadequate in our devices to protect the solder glass bykeeping it about 1 mm back from the discharge volume;the exact necessary setback should in general be de-pendent on seal thickness and field intensity.

Life TestsIn Fig. 10 are shown the life test records of our lon-

gest-lived pressed glass laser. This device is still run-ning as the present paper is being written. More re-cently manufactured devices have not been extensivelylife tested because of a need to devote most of ouravailable life test facilities to tubular lasers.

This particular laser was relatively dirty as it cameoff the gas-processing station. Emission spectroscopyshowed strong carbon monoxide and oxygen contami-nation. The contaminants cleaned up after about 10days of operation.


Fig. 9. P

1.2

E 1.0

D 0.8

3 0.6

w 0.4

2 0.2

6E4

Xs 2U .

1400

01300

a 6

Va 5cc: ra

01~-

I 3I-Za 2

Fig. 10.output; (1Hastingstuations

CUILY 1JUELUKIIIINg 111CUM.

The 13,000 h achieved thus far we take as indicativethat lasers of this general type have sufficient promisefor further development. A reasonable product-de-velopment goal would be to verify that 10,000-h devicescan reliably be manufactured in quantity. Consider-able further effort will- be required to reach thispoint.

Other Construction OptionsFigure 7 suggests a few of the alternative construction

methods which are compatible with the integrated-laserapproach. The device of Fig. 7(a) is designed for usewith an external mirror; one end of the laser bore isclosed with a Brewster window, which polarizes theoutput beam. It would also be possible to put a Brew-ster window in an internal slot in a two-mirror laser.

Figure 7(a) also illustrates that a laser body need haveno drilled holes; the vacuum-processing tubulation issealed into a groove in one substrate, and the anode isa thin platinum foil sealed between the substrates.

The device of Fig. 7(b) (which was invented by ourcolleague A. D. Lewis) illustrates the notion of usingmore than two glass parts to get a rather simple con-struction. The laser bore is defined between the two6.4-mm float glass pieces, and the cathode (of tubularaluminum, not thin film in this case) is sealed in thetumblerlike third part, along with a getter.

ConclusionWe have demonstrated the feasibility of building a

long-lived gas laser from pressed-glass parts and feelthat development of a production prototype will bewarranted when markets appear for hundreds ofthousands of identical units, which may occur short-ly.

We are also intrigued by some special capabilities ofthe pressed-glass and flat-glass approaches, whichmight warrant product development for smaller mar-kets:

(1) These structures are well suited for transverse-discharge lasers. Film electrodes can easily be appliedto the upper and lower substrates and can be coveredwith insulating layers if one wishes to take advantageof charge storage on insulating surfaces to reduce thenet drive voltage needed.

(2) Multiple-bore or multiple-pass lasers can easilybe constructed, with the several bore segments in ex-cellent and stable alignment, simply by manufacturingseveral grooves in a common substrate. (A similar ap-proach has been suggested by Andringa9 for construc-tion of two bores of a ring laser in a single substrate.)

(3) Integrated optical systems can easily be con-structed, with a laser and related optical componentsrigidly mounted in or upon a single substrate.

(4) It is straightforward, with our approach, to pro-vide precisely oriented alignment surfaces, flatmounting surfaces, holes for bolts, and the like, whichought to make it somewhat easier to mount and alignour lasers in a system than to mount and align conven-tional tubular lasers.

(5) Our design makes all interior surfaces of the laserdevices readily accessible during manufacture. Lasers

requiring coated surfaces, complex interior surfaceconfigurations, or internally mounted components cantherefore be manufactured with greater ease thanheretofore.

(6) Although the economics of high-volumepressed-part manufacture strongly favor the use ofsoda-lime glass, the integrated-laser approach shouldalso be applicable with lower-expansion glasses. Solderglasses are even available for use with zero-expansionmaterials such as Owens-Illinois Cer-Vit. Thus, we cancontemplate the manufacture of frequency-stable laserswith this new family of methods.

(7) Lasers fabricated from pressed or flat glass appearto be inherently more rugged than conventional tubulardesigns, which might make them desirable for militaryapplications.

The successful operation of thin-film cathodes mayprove to be an independently useful contribution to thelaser art. In contrast to sheet-metal cathodes, thin-filmcathodes require no support means other than the wallof the cathode cavity, do not require procedures suchas machining in water 0 to remove the surface foulingleft by the extrusion of aluminum tubing, do not requirehigh-current processing for degassing, and are com-patible with high-volume in-line fully automatic man-ufacturing equipment.

Construction of the integrated laser could not haveoccurred without the frequent advice and assistance ofthe dozens of people involved in O-I's tubular-laserdevelopment effort and numerous members of the GlassForming Engineering Department. There are too manyto thank them all by name, and we hope our gratitudewill seem no less sincere because we must recognizethem en masse.

Several men must receive special thanks for the greatamount of time and patience they invested in thisproject. Our many early flat-glass prototypes werefabricated by Peter Gerhardt and David Kuebler.Daniel Brackette and Fred Acampora processed andtested many of our devices and assisted us greatly in thedevelopment of workable procedures. Robert Kniselyhad primary responsibility for design and manufactureof pressed glass parts. Leonard Roth, manager of O-I'sgas laser development effort, was a consistent source ofassistance and encouragement.

References1. U. Hochuli and P. R. Haldemann, Gas Laser, U.S. Patent 3,614,642, (19 October

1971).2. J. P. Goldsborough, in Laser Handbook, F. T. Arecchi, Ed. (North-Holland, Amsterdam,

1972), Pp. 597ff.3. G. Francis, "The Glow Discharge at Low Pressure," in Handbuch der Physik, S. Flugge,

Ed. (Springer-Verlag, Berlin, 1956), Vol. 22.4. cf. 0. A. Boyarchikov and A. S. Shipalov, T. Mosk. Energ. Inst. Radioelektron. No. 108,

89 (1972). nglish translation available from National Technical Information Serviceasl AD771885.

5. J. Gallup, J. Am. Ceram. Soc. 29, 277 (1946).6. E. J. Panner, "Catastrophic Early Life Electron Tube Failure as a Result of Electrolytic

Curents Flowing Through the Gilaas Bulb," in Advances in Electron Tube Techniques(Proceedings of the Fifth National Conference), David Sister, Ed. (Pergamon, NewYork 1961).

7. R. H. Dalton, "Electrolysis and Polarization in Glass," in Comptes Rendus VIIe CongrgsInternational du Verre, Bruxelles (Institut National du Verre, Charleroi, Belgique,1965).

8. cf. D. C. Sinclair and W. Earl Bell, Gas Laser Technology (Holt, Rinehart, New York,1969).

9. K. Andringa, Laser gyroscope, U.S. Patent 3,930,731 (1976).10. U. Hochuli, P. Haldemann, and D. Hardwick, IEEE J. Quantum Electron. QE-3, 612

(1967).


Documents

Integrated construction of low-cost gas lasers